1. Field of the Invention
This invention relates to ink jet printers, such as are typically used with computers and scientific instruments.
2. Description of the Prior Art
Ink jet printers, one type being thermal ink jet printers, are well known in the art, and are commonly used with personal computers and with scientific instruments. When used with scientific instruments, the printers are often called "printer-plotters". Ink jet printers are made by several companies. Hewlett Packard's family of portable ink-jet printers is a well known example. Thermal ink jet printing uses thermal excitation to fire (i.e., eject) drops (also called dots) of ink through tiny orifices, to print text or pictures. FIG. 1 shows characters printed by means of such ink dots.
A key component of the ink jet printer is the ink jet cartridge. The Hewlett Packard ink jet cartridge is one type. It is a disposable unit 20 which is total self-contained (FIG. 2). The cartridge consists of a liquid ink supply in a bladder 21, twelve nozzles 22a, 22b, etc., and twelve thin film resistors (not shown). The resistors are located directly below each nozzle 22a, 22b, etc. Each nozzle 22a can supply a drop of ink on demand by energizing the corresponding resistor. The drop ejection process begins by heating the resistor with a short electrical pulse. Within a few microseconds, the ink above the resistor is vaporized. The vapor bubble grows rapidly and imparts momentum to the ink above the bubble. Some of this ink is ejected through the nozzle 22a at velocities exceeding ten meters per second. The nozzle 22a is then automatically refilled with ink by capillary action.
The ink supply is contained in a synthetic rubber bladder 21 located immediately behind the printhead substrate 23. The bladder 21 is designed to maintain a relatively constant back pressure at the nozzles 22a so ink is only expelled when desired. The bladder 21 also provides a very crude visual indication of the amount of remaining ink, because the bladder 21 collapses as ink is used. Ink flow from the bladder 21 to the nozzles 22a is by capillary action, and is relatively independent of the print cartridge 20 orientation. The bladder 21 contains enough ink to print some approximate number of dots, about ten million dots in the case of the Hewlett Packard cartridge. The prior art method of determining when the bladder 21 is out of ink is simple. If the print cartridge 20 fails to print, and the bladder 21 looks collapsed, the print cartridge 20 is out of ink and needs replacement. Priming the print cartridge (i.e., quickly printing several dots from each nozzle) may temporarily restore printing (because of a small amount of residual ink in the bladder 21), but printing will only continue for a few hundred more characters.
The cartridge 20 also includes locating pins 24, cover 25, resistor array electrical contacts 26a, 26b, etc., and body 27. The prior art printers are typically controlled by a computer program installed in ROM (Read Only Memory) in a microcontroller in the printer.
In its product literature, Hewlett Packard suggests an alternate procedure for detecting loss of print (i.e., ink exhaustion), by using an optical sensor, presumably by sensing a loss of dark (printed) areas on the paper. No details are provided.
However, this prior art method does not provide any warning before the ink supply is exhausted. In the typical personal computer application, loss of ink supply is not so serious, since each document is usually short and the printer is under operator control. When the printer is used in an industrial or scientific application such as for recording data from instrumentation and operates unattended, loss of printing is a significant problem.
Therefore, there is a need for a way to anticipate the exhaustion of the ink supply in the bladder, so as to warn the operator to install a fresh cartridge.